For example, with regard to business cards given by others, a file of plastic film with pockets in which to keep cards is commercially available. Such a file, however, has a much greater size than each card and, in addition, is relatively high in price. Also, it is not possible to view the reverse sides of the cards kept in such a file.
On the other hand, it is possible to keep the cards compactly by holding them together as if to bind a book. Also, if the cards are held together in such a manner, their reverse sides may be viewed. However, the work of holding together cards or similar materials has so far required an expensive instrument and expertise. So far, no device for holding such materials as if to bind a book has been available for personal use.